The present invention relates to a metallic gasket that comprises a base plate having a bead around the periphery of a combustion chamber hole therein and a thinner shim plate than the base plate laminated on the base plate.
The prior-art metallic gaskets of this type, for example, include a base plate a having a combustion chamber hole b therein and a thinner shim plate d than the base plate laminated on a limited area of the base plate around the periphery of the combustion chamber hole b (present on the combustion chamber hole side of a base plate bead c), as shown in FIG. 21; two base plates a and a shim plate d interposed between them, as shown in FIG. 22; and two base plates a, a shim plate d and a secondary plate e interposed between the two base plates a with an edge of the shim plate d on the combustion chamber hole b side folded back so as to cover an edge of the secondary plate e, as shown in FIG. 23.
In any of these metallic gaskets, the limited area around the periphery of the combustion chamber hole b is formed so as to have a maximum total thickness compared to the other areas of the gasket such that when the metallic gasket is interposed between joining surfaces of the cylinder head and the cylinder block, and the cylinder head and the cylinder block are fastened by the fastening bolts, a surface pressure is concentrated on the limited area around the periphery of the combustion chamber hole b to thereby apply a maximum load to the periphery of the rim of the cylinder bore to seal.
FIG. 24 shows a recent aluminum cylinder block employed frequently by a demand for a reduction in the engine weight. The cylinder block 1 has a cylindrical cast-iron sleeve 2, for example press-fitted into a cylinder bore in the cylinder block 1 and supported at its lower end by a step 3 formed on the cylinder block 1 from a standpoint of durability and wear resistance of the bore. In order to enhance the cooling efficiency of the engine by cooling water in water jackets 4 formed in the cylinder block 1, the sleeve 2 is thinned as much as possible (to about 2-5 mm) with the aid of the advancement of recent working techniques or the like.
However, when the metallic gasket is interposed between the joining surfaces of the cylinder block 1 and the cylinder head (not shown), and the cylinder block 1 and the cylinder head are fastened by the fastening bolts, the following problems would occur:
The cast-iron sleeve 2 on the bore side is heated to high temperature during running to thereby conduct heat to the aluminum cylinder block 1 in contact with the sleeve 2, which is thus cooled. Since an extremely large amount of heat is produced on the cylinder bore side, the sleeve 2 that has no cooling structure would axially expand thermally due to a difference in temperature and hence cubical expansion between the sleeve 2 and the cylinder block 1 to thereby intend to protrude outwards from the deck of the cylinder block 1.
Therefore, with the metallic gasket comprising the thickened area that includes the limited area around the periphery of the combustion chamber hole b formed therein and the shim plate d laminated on the limited area such that the maximum load is applied to the periphery of the rim of the cylinder bore, the thickened area is pushed by an upper end of the sleeve 2 to thereby produce an excessive surface pressure in the vicinity of the fastening bolts especially in a high surface-pressure area around the periphery of the sleeve 2. As a result, the step 3 on the cylinder block 1 supporting the lower end of the thin sleeve 2 would be broken undesirably. In addition, the sleeve 2 would be deformed due to an extremely uneven surface-pressure in the periphery of the sleeve (in the areas in the vicinity of and between each bolt) to thereby impair the roundness of the cylinder bore.
Even when the step 3 on the cylinder block 1 is not broken, the sleeve 2 could sink when the area of the step 3 is small. In addition, when the cylinder head has low rigidity, the sleeve 2 could push the cylinder head up to thereby cause pressure leakage from between the fastening bolts.
Instead of the engine with the cast-iron sleeve pressed into the cylinder bore as described above, so-called all-aluminum engines have recently started to be employed in which the aluminum cylinder block has a directly formed bore whose inner peripheral surface (aluminum surface) is coated by plating or exposure with a film of metal, ceramic or resin to ensure durability.
Such all-aluminum engine is capable of avoiding the defects of the cast-iron sleeve mentioned above. It is lightweight and has high thermal conductivity. Therefore, this improves the cooling efficiency of the engine by the cooling water within the water jackets of the cylinder block.
With the all-aluminum engine, however, the periphery of the rim of the cylinder bore has low rigidity compared to the cylinder block into which the cast-iron sleeve is press fitted. Thus, with the metallic gasket that has formed a thickened area around the periphery of the combustion chamber hole b by laminating the shim plate d on the base plate around the periphery of the combustion chamber hole b so as to apply the maximum load to the periphery of the rim of the cylinder bore, the periphery of the rim of the cylinder bore would be deformed due to the maximum load to thereby make it difficult to ensure the roundness of the cylinder bore.
The present invention has been made to eliminate such defects. It is an object of the present invention to provide a metallic gasket capable of providing sufficient sealing performance through the overall periphery of the combustion chamber hole and when used with an engine into which the cast-iron sleeve is press fitted, avoiding difficulties occurring due to axial thermal expansion of the cast-iron sleeve during running, i.e., breakage of the step on the cylinder block supporting the sleeve at its lower end, impairment of the roundness of the cylinder bore, sinking of the sleeve, and occurrence of pressure leakage from between the fastening bolts, and when used in the all-aluminum engine, capable of ensuring the roundness of the cylinder bore well.
The present invention provides a metallic gasket comprising a base plate having a bead around a combustion chamber hole formed therein, and a thinner shim plate than the base plate laminated on the base plate, the metallic gasket being adapted to be interposed between joining surfaces of a cylinder head and a cylinder block and fastened by fastening bolts through the cylinder head and cylinder block to seal between the joining surfaces, wherein:
an edge of the shim plate on the combustion chamber hole side is located radially outside the combustion chamber hole in spaced relationship to the combustion chamber hole and an edge of the shim plate distant from the combustion chamber hole is located on the combustion chamber hole side from bolt insertion holes in the base plate formed outside the bead so as to avoid a water hole formed between each of the bolt insertion holes and the bead.
According to the present invention, the surface pressure is concentrated on the periphery of the combustion chamber hole due to the difference in thickness between a thickened area of the periphery of combustion chamber hole laminating the shim plate and the other thinner areas of the gasket. Thus, the maximum load works on the periphery of the rim of the cylinder bore where the sealing conditions are most severe. In addition, a possible high-temperature high-pressure combustion gas is sealed by the sealing pressure based on a resiliency of the compressed deformed bead to thereby provide sufficient sealing performance through the overall periphery of the combustion chamber hole.
Since the edge of the shim plate on the combustion chamber hole side is located radially outside the combustion chamber hole, no maximum load works on the low-rigidity periphery of the rim of the cylinder bore. As a result, when the gasket is used with the all-aluminum engine, the periphery of the rim of the cylinder bore is prevented from being deformed to thereby ensure the roundness of the cylinder bore sufficiently.
In this case, the thin plate may be disposed such that an edge portion of the shim plate on the combustion chamber hole side between the bolt insertion holes is located close to the combustion chamber hole and an edge portion of the shim plate in the vicinity of each bolt insertion hole is located less close from the combustion chamber hole to thereby make uniform the surface pressure that works on the overall periphery of the rim of the cylinder bore.
When the gasket is used in an engine where a cast-iron sleeve is fitted into the cylinder block, the edge of the shim plate on the combustion chamber hole side may be located radially outside the outer periphery of an upper end of the cast-iron sleeve and at least part of the bead may be disposed on the upper end of the sleeve in order to advantageously avoid difficulties that would otherwise occur due to thermal axial expansion of the sleeve during running, i.e., breakage of the step on the cylinder block supporting the lower end of the sleeve, damage to the roundness of the cylinder bore, sinking of the sleeve, and pressure leakage from between the fastening bolts.
In addition, with an engine where the cast-iron sleeve is pressed and the space between the cylinder bores is large, a metallic gasket including a plurality of beads spaced one from another in a radially outward direction from the combustion chamber hole may be used such that a part or all of that of the plurality of beads nearest to the combustion chamber hole is disposed on an upper end of the cast-iron sleeve pressed into the engine.